A known twofold seat back of a seat apparatus for a vehicle, which is disclosed in JP2008-254645A (hereinafter referred to as Reference 1, see paragraphs 0006, 0017, and 0018 and FIG. 4) is divided into an upper seat back portion supporting a seated occupant's shoulder blade and a lower seat back portion supporting the seated occupant's lumbar part. The upper seat back portion includes upper frames each serving as a first seat member and the lower seat back portion includes lower frames each serving as a second seat member. Each of the upper frames of the upper seat back portion and each of the lower frames of the lower seat back portion are rotatably connected at respective sides thereof to each other by a hinge shaft. A seat back angle adjustment mechanism (seat back adjustment mechanism) is arranged between the upper and lower frames and in a position deviated rearward from the hinge shaft. The seat back angle adjustment mechanism includes ball screws (extending and retracting device), guide members (extending and retracting device), a shaft (drive device), operating members (extending and retracting device), and a drive motor (drive unit) and adjusts a tilt angle of the seat back upper portion relative to the seat back lower portion.
The guide members are rotatably connected to the both side ends of the lower frame of the lower seat back portion by means of hinge pins. Each of the ball screws is screwed with each of the guide members so as to be rotatable. The ball screws are connected to each other by the shaft arranged therebetween so as to rotate in a synchronized manner. The operating members are rotatably connected to the both side ends of the upper frames of the upper seat back portion by means of hinge pins. An upper end portion of each of the ball screws is rotatably screwed with each of the operating members. Further, the drive motor driving the ball screws is attached to one of the guide members. According to the seat back angle adjustment mechanism configured as described above, the drive motor is driven to linearly move the ball screws in a vertical direction of the vehicle, thereby rotating the upper frames of the upper seat back portion about the hinge shafts. Accordingly, the upper seat back portion tilts relative to the lower seat back portion.
A known seat back of a seat apparatus for a vehicle, disclosed in JP2007-006948A (hereinafter referred to as Reference 2, see paragraph 0015, FIGS. 2 and 3) includes a twofold seat back having a seat back upper portion and a seat back lower portion. Further, the seat back is provided with a seat back tilting mechanism (seat back adjustment mechanism). The seat back tilting mechanism includes a feed screw (extending and retracting device), a nut (extending and retracting device), a housing (drive unit), an arm member (drive unit), and a motor (drive unit). The housing is fixed to the center of a lower support member of the lower seat back portion by a bracket. The nut is rotatably supported within the housing. The feed screw is rotatably screwed with the nut. The arm member is fixed to the center of an upper support member of the upper seat back portion by a bracket. An upper end portion of the feed screw is rotatably screwed with the arm member. Further, a motor rotating the nut is attached to the housing. According to the seat back tilting mechanism configured as described above, the motor is driven to linearly move the feed screw in a vertical direction of the vehicle, thereby rotating the upper seat back portion around hinge shafts rotatably connecting the upper and lower seat back portions at both sides thereof. Accordingly, the upper seat back portion tilts relative to the lower seat back portion accordingly.
In case of a collision of the vehicle, a large load is applied to the upper frame of the upper seat back portion due to an inertia force of the occupant in a forward or rearward moving direction of the vehicle. The seat back adjustment mechanism described in Reference 1 and Reference 2 is configured so that the upper frame of the upper seat back portion is rotated around the hinge shafts by a screw mechanism. Accordingly, in case of a front collision of the vehicle, a moment force, which is obtained by multiplying a load applied to the upper frame of the upper seat back portion by a distance between the upper frame and each of the hinge shafts acts on the hinge shaft. Further, a load obtained by dividing the moment force by a distance from the screw mechanism to the hinge shaft is applied to the screw mechanism.
In recent years, a longitudinal thickness of a seat for the vehicle is desired to be small because of an improvement of the appearance of the seat. Accordingly, the distance from the screw mechanism to the hinge shaft is designed to be short. In general, a ratio between the distance from the upper frame to the hinge shaft and the distance from the screw mechanism to the hinge shaft is approximately from seven to ten to one; therefore, the load applied to the screw mechanism is from seven to ten times as large as the load applied to the upper frame. Consequently, the screw mechanism requires a strong configuration that is not damaged even when a large load is applied to the screw mechanism. This increases the cost and weight of the seat back adjustment mechanism. In addition, the screw mechanism needs to generate a large driving force in order to tilt the upper seat back portion. Accordingly, the screw mechanism may additionally require a speed reduction gear mechanism performing a large speed reduction, therefore further increasing the cost weight of the seat back.
A need thus exists for a seat apparatus for a vehicle and a seat adjustment mechanism for the same, which are not susceptible to the drawback mentioned above.